1. Field of Invention
The present invention relates to the semiconductor field, and specifically to a miniature thermoelectric energy harvester and a fabrication method thereof.
2. Description of Related Arts
With the development of wireless network sensor technology, its applications in the industrial, commercial, medical, consumption and military fields are gradually expanded. Power source is always critical to prolonging the service life and reducing the cost of wireless network sensors. In environmental extremes or other occasions unreachable to human beings, or when a network node moves or changes, it is difficult or even impossible to replace a battery, making it crucial to effectively provide energy to a wireless network sensor. An effective solution is to harvest ambient energy through energy harvesting, store the energy and provide the energy to the wireless network sensor. Currently, the most commonly used energy harvesting method is to use the Seebeck effect to convert a temperature difference in an environment to electrical energy for energy harvesting. On the other hand, as system miniaturization leads to decreasing system size and power consumption, energy required for system operation also decreases; therefore, a thermoelectric chip may be used to harvest ambient energy so as to supply power to the system.
Current miniature thermoelectric energy harvesters are mainly classified into two types, that is, planar type and vertical type. FIG. 1A is a schematic structural view of a planar-type miniature thermoelectric energy harvester, wherein the direction of heat flow is parallel to the substrate, and thermocouples are arranged parallel to the substrate. Since the planar-type thermoelectric energy harvester is in contact with the ambient environment through cross sections of its components, there is a small contact area between the thermoelectric energy harvesting chip and the environment, leading to undesirable thermal contact between the thermoelectric energy harvesting chip and the environment, and affecting the operating efficiency of the thermoelectric energy harvesting chip. However, the thermocouples on the substrate are generally fabricated by a planar semiconductor process, and the thermocouple length is 1-1000 μm dependent upon the photolithographic process, and the thermocouple length can be controlled through pattern design. In addition, to increase the thermal resistance of the thermoelectric energy harvesting chip, the thermocouple structure generally needs to be thermally insulated, that is, the substrate below the thermocouple is hollowed out. As a result, the thermocouple structure is eventually suspended on the substrate, with a cross-sectional structure as shown in FIG. 1B. Since the thermocouple is a suspended structure, the thermocouple microstructure is easily broken, degrading the reliability of the chip.
FIG. 1C is a schematic structural view of a vertical-type miniature thermoelectric energy harvester, where the direction of heat flow is perpendicular to the substrate, and thermocouples are arranged perpendicular to the substrate. Since the vertical-type thermoelectric energy harvester is in contact with the ambient environment through the whole substrate, there is a large contact area between the thermoelectric energy harvesting chip and the environment, so that good thermal contact is achieved between the thermoelectric energy harvesting chip and the environment, thereby improving the operating efficiency of the thermoelectric energy harvesting chip. However, since the thermocouples are arranged perpendicular to the substrate, the planar semiconductor process cannot be adopted, and instead, the thermocouples are generally fabricated by an electroplating or thin-film sputtering deposition process, resulting that the thermocouple length is limited by the process. The thickness of a film fabricated by the thin-film sputtering deposition process is generally smaller than 100 μm, while the thickness of a film fabricated by the electroplating process is generally smaller than 1000 μm. Current vertical-type thermoelectric energy harvesting chips generally adopt a BiTe-based material or a metal material such as Cu or Ni as the thermoelectric material. Since the metal material such as Cu or Ni has a small Seebeck coefficient, thermoelectric energy harvesting chips fabricated by using the metal material such as Cu or Ni as the thermoelectric material generally have low efficiency. Since the BiTe-based material has a high Seebeck coefficient, thermoelectric energy harvesting chips fabricated by using the BiTe-based material generally have high efficiency. However, the BiTe-based material requires a high cost, and contains toxic substances, which limits the use of BiTe thermoelectric energy harvesting chips. In addition, since the composition of a thermocouple requires two thermoelectric materials, the vertical-type thermoelectric energy harvester generally needs to be subjected to two electroplating or thin-film sputtering deposition processes in order to fabricate a thermocouple material, which further increases the cost of the thermoelectric energy harvesting chip. Moreover, the fabrication efficiency of the vertical-type thermoelectric energy harvester is low, because the thermoelectric energy harvester is thermally and mechanically connected to upper and lower substrates through chip-level bonding.